The present invention relates to an apparatus for heat-fixing toner image onto papers or other sheets in image-forming apparatuses such as electrophotographic apparatuses and electrostatic printing apparatuses, and more particularly to a heat-fixing apparatus suitable for printers for printing continuous papers such as rolled papers and folded papers as output apparatuses of computers.
Conventionally used to fix toner image onto recording media or carrier sheets such as zinc oxide photoconductive papers, plain papers, plastic sheets, etc., is a heat-fixing apparatus comprising a heat roll and a pressure roll rotatably disposed opposite to each other in a pressed contact state, through which the carrier sheets bearing toner image pass while being heated under pressure. In such an apparatus a pressure roll having an outer surface layer made of a heat-resistant elastic material is disposed opposite to the heat roll and kept in a pressed contact state to have a predetermined contact width or nip width between the two rolls. Such a heat-fixing apparatus is used in conventional electrophotographic apparatuses, for instance, copying machines, in which the heat-fixing apparatus is in cooperation with an electrostatic image-developing apparatus for fixing toner image onto cut papers. In recent years, they are used as printers for fixing toner image onto continuous papers such as rolled papers or folded papers in cooperation with the output apparatuses for computers.
In a heat-fixing apparatus using continuous papers, to stop fixing operation, a heat-generating source and a driving source should be inactivated, and a continuous paper should be separated from the heat roll. That is, unlike a heat-fixing apparatus using usual cut papers, mere inactivation of the heat-generating source and the driving source leads to undesirable heating, and scorching in extreme cases, of the continuous paper by the heat roll, because the continuous paper is kept between the heat roll and the pressure roll, so that it is heated by the heat roll. Accordingly, the pressure roll should be separated from the heat roll in a non-printing period. However, mere separation of the pressure roll from the heat roll does not necessarily enable the continuous paper to be separated from the heat roll.
FIG. 2 is a schematic view showing a conventional heat-fixing apparatus. In this figure, 1 and 2 respectively denote a heat roll and a pressure roll, which are rotatable in a pressed contact state. It comprises an introduction guide 3 on the upstream side of the pressure roll 2 and a pair of paper-discharging rolls 4 rotatable in a pressed contact state disposed on the downstream side of the pressure roll 2. 5 denotes a discharge guide.
By the above structure, a continuous paper 6 bearing toner image is introduced between the heat roll 1 and the pressure roll 2 by the introduction guide 3 to carry out the fixing of toner image, and then discharged via the discharge guide 5 and a pair of the paper-discharging rolls 4. In the case of stopping the fixing operation, namely in a non-printing period, the heat roll 1 and the pressure roll 2 are separated vertically. However, the continuous paper 6 is not separated from the heat roll 1 because of the structure of the heat-fixing apparatus and physical attraction and adhesion between the heat roll and the continuous paper 6. Specifically speaking, since a downstream end 3a of the introduction guide 3, a gap between the heat roll 1 and the pressure roll 2 and a gap between a pair of the paper-discharging rolls 4 are substantially on the same plane, the continuous paper 6 cannot be released from the heat roll 1 only by moving the pressure roll 2 downward. Also, since the continuous paper 6 is electrostatically charged in the preceding image transfer process, it is likely to be attracted to the heat roll 1. In addition, there is adhesion due to toner between the continuous papers 6 and the heat roll 1. Accordingly, the continuous paper 6 is still kept in contact with the heat roll 1 even when the pressure roll 2 moves downward to permit the continuous paper 6 to sag.